The worst? You might void the warranty, I suppose. Since I would have a hard time blaming the manufacturer for my (perceived) screw-ups in the winemaking process, I've never considered exercising the warrant y anyway, so that's not an issue for me.
As a practical matter, the instructions, in substance, vary only a little. Some have you re-hydrate the yeast, while others have you sprinkle it without re-hydrating. Some have you rack from the primary a little earlier than others. Some have you bottle a few days sooner than others. Most of it is timing, so not really much difference, IMHO.
I have made enough kits that I don't usually bother with the instructions unless there is something out of the ordinary (two yeast packets, multiple oak packets, dried wine skins AND f-pack, etc.). So, if you feel you have a good handle on the process, don't sweat the small stuff - make the wine the way you are comfortable, is my advice.